1. Field of the Invention
The present invention relates to a laser apparatus and method for producing a high quality, low divergence optical beam having a wavelength shifted from that of an input optical pump beam by a Stokes shift due to stimulated Raman scattering (SRS).
2. Description of the Related Art
Laser sources for various applications are required to emit high quality, coherent beams at a wavelength which will not damage the human eye. The wavelength of 1.54 microns has been generally established as being "eye-safe". However, it is difficult to directly produce a laser beam at this wavelength using conventional lasers.
A Nd:YAG laser in widespread use is capable of producing a high quality beam at 1.06 microns, which is outside of the eye-safe range. Raman laser devices utilizing the Stokes shift in a Raman scattering medium can be used to convert laser radiation of one wavelength to a longer wavelength. Methane is a Raman medium having a vibrational Stokes frequency shift of 2916 cm.sup.1, enabling conversion of a 1.06 micron beam to a 1.54 micron beam This conversion may be accomplished as described, for example, in U.S. Pat. No. 4,821,272, entitled "SINGLE MIRROR INTEGRAL RAMAN LASER", issued Apr. 11, 1989 to H. Brusselbach et al, and assigned to the same assignee as the present application.
Raman scattering of an input optical beam in a suitable medium produces both forward and backward propagating SRS waves, as described in a basic treatise on stimulated Raman scattering found in "Tunable Lasers", by J. C. White, Springer Series Topics in Applied Physics, Vol. 59, Springer, Berlin, Heidelberg, 1987, pp. 115-207. The arrangement disclosed in the Brusselbach patent is known as a "Raman half-resonator", and produces a Ramanshifted output using the forward SRS wave. The beam divergence of this type of laser is on the order of twice the pump beam divergence, which is undesirable for certain applications.
The backward SRS wave is retro-reflected back onto the input pump beam. This reduces the number of optical elements that affect output beam alignment in a backward Raman laser. In addition, the backward SRS wave does not parametrically couple to anti-Stokes radiation, whereas the forward SRS wave does. This coupling of the SRS wave to anti-Stokes reportedly causes an effective reduction in gain for the lower order modes of the forward SRS wave, and therefore increases forward Raman beam divergence, as discussed, for example, in the publication by Perry et al., "Stimulated Raman Scattering With a Tightly Focused Pump Beam", Optics Letters, Vol. 10, No. 3, 146 (1985).